diff --git a/library/ssl_cookie.c b/library/ssl_cookie.c
index cb89c94..dd46971 100644
--- a/library/ssl_cookie.c
+++ b/library/ssl_cookie.c
@@ -227,7 +227,7 @@
     if( ret != 0 )
         return( ret );
 
-    if( mbedtls_cf_memcmp( cookie + 4, ref_hmac, sizeof( ref_hmac ) ) != 0 )
+    if( mbedtls_ct_memcmp( cookie + 4, ref_hmac, sizeof( ref_hmac ) ) != 0 )
         return( -1 );
 
 #if defined(MBEDTLS_HAVE_TIME)